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Newton’s Third Law
Actions & Reactions
Newton’s Third Law of
Motion
Forces are not “one-sided”.
Whenever one object exerts
a force on a second object,
the second objects exerts a
force back on the first
object.
The force exerted by the second
object is equal in strength and
opposite in direction of the first
force. The first force is called
the “action” and the other force
is called the “reaction”.
Newton's third law of
motion describes the
relationship between these
two forces. Newton's third
law of motion states that if
one object exerts a force on
another object, then the
second object exerts a force
of equal strength in the
opposite direction on the
first object.
Another way to state Newton's
third law is that for every
action there is an equal but
opposite reaction.
Action - Reaction Pairs
Pairs of action and
reaction forces are all
around you. When you
jump, you push on the
ground with your feet.
This is an action force.
The ground pushes back
on your feet with an
equal and opposite force.
This is the reaction force.
Detecting Motion
You cannot always detect
motion when pared forces
are in action. An example of
this is when Earth's gravity
pulls on an object, you
cannot detect Earth's equal
and opposite reaction.
You don't see Earth accelerate
toward the object because
Earth's inertia is so great that
its acceleration is too small to
notice.
Do Action - Reaction
Forces Cancel?
Newton's third law refers to
forces on two different
objects.
The action and reaction forces
described by this law cannot
be added together because
they are each acting on a
different object.
Forces can be added together
only if they are acting on the
same object.
For example, if a volleyball player
hits a ball, she exerts an action
force on the ball. In return, the
ball exerts an equal but opposite
reaction force back on her hands.
The action and reaction forces
act on different objects.
But if two volleyball players both
exert a force on the same object –
the volleyball – when they hit the
ball from opposite directions, each
of their hands exerts a force on the
ball equal in strength but opposite
in direction. The forces on the
volleyball are balanced and the ball
does not move in either direction.
Momentum
All moving objects have what
Newton called a “quantity of
motion”. We call it momentum.
Momentum is a characteristic of
a moving object that is related
to the mass and the velocity of
the object.
The momentum of an object
can be determined by
multiplying the object's mass
and its velocity.
Momentum = Mass X Velocity
The unit for momentum is
kilogram-meters per
second (kg.m/s), since mass
is measured in kilograms
and velocity in meters per
second. Like velocity and
acceleration, momentum is
described by its direction
in addition to its quantity.
The momentum of an object is
in the same direction as the
velocity of the object. The more
momentum a moving object
has, the harder it is to stop.
Conservation of Momentum
The word conservation has a
special meaning in physical
science. In everyday language,
conservation means saving
resources. You might
conserve water or electricity.
In physical science,
conservation refers to the
conditions before and after
some event. An amount that is
conserved is the same amount
after an event as it was before.
The amount of momentum objects
have is conserved when they
collide. When two objects collide
in the absence of friction,
momentum is not lost.
The law of conservation of
momentum states that, in the
absence of outside forces, the
total momentum of the
objects that interact does not
change. It is the same before
and after the interaction.
A quantity that is conserved
is the same after an event as
it was before an event.
The total momentum of any
group of objects remains the
same, or is conserved, unless
outside forces act on the
objects. Friction would be an
example of an outside force
that might act on the objects.
Collisions with
Two Moving Objects
Momentum is conserved
when two objects, such
as trains, collide.
If one train traveling fast
collides with a slowermoving train on the same
track, the faster train
slows down, and the
slower train speeds up.
Collision with
Connected Objects
If a moving train
collides and locks with
a train at rest, both cars
will then move, but they
will move more slowly
than the first car did.
In each of these
examples, momentum is
conserved.
Collision with
One Moving Object
If a moving train collides with a train at rest, the first train stops moving
and the second train begins to move.
All of the momentum has been transferred to the car that was at rest.
Keywords: English - Spanish
Newton’s Third Law of Motion
La tercera ley de movimiento de Newton
Momentum - Impulso
Conservation - Conservación
Conservation of Momentum - Conservación de Momento